Principal Investigator Raul Radovitzky
Co-investigator Michael J Demkowicz
Project Website http://isnweb.mit.edu/project-3.2.1.html
This project proposes two task lines related to alloys with unique, engineered grain boundary structures.
(1) Lightweight nanocrystalline alloys exhibit many ideal properties of direct interest for protection applications, such as high specific strength, strain rate sensitivity, and the ability to dissipate impact energy through plastic deformation. Unfortunately, there is not generally a single grain size that simultaneously optimizes all of these properties. For example, nanostructured materials with a uniform grain size of about 10 nm are known to optimize strength and rate sensitivity, but do not necessarily optimize strain hardening capacity or toughness. For protection applications, where multiple properties require simultaneous optimization, it is desirable to pursue hybrid microstructures with nano to micro scale grain sizes and multimodal grain size distributions. The present research proposes to initiate a systematic study on the fabrication, characterization, and mechanical testing of a new class of nanocrystalline materials with an additional hierarchical level of structure: layered and graded nanocrystalline materials. Building on successful research on monolithic Al-Mn nanocrystalline alloys in prior ISN-supported work, we propose to synthesize aluminum based alloys with hybrid nano structures in the form of multilayers using electrodeposition. Theoretical and computational modeling will be carried out to guide the experimental challenge in designing nanostructured alloys that are not only stable, but in which the nanostructure can be controlled and patterned at higher length scales. The deposit microstructures will be tuned by changing alloying composition and deposition parameters for optimized mechanical properties of interest in vehicle and personnel protection.
(2) The project builds on the discovery that so-called "oligocrystalline structures" of shape memory alloy (SMA), i.e., those which have a high amount of surface area relative to the area of grain boundaries within them, exhibit remarkably enhanced energy dissipation and improved flexibility. These oligocrystalline structures are characterized by small dimensions, such as for fine diameter fibers and wires, or foams with fine struts. In the proposed work we will develop the basic processing science needed to produce flexible SMA fibers for useful structures such as cables, ropes and woven garments with focus on soldier applications. We will also adapt the processing parameters to tune the shape memory and damping properties to meet various application requirements. Simultaneously, we will continue our modeling efforts in order to gain additional insight in how the surface-dominated physics of the oligocrystallline state may best be plied to useful purposes in increasing the survivability of the soldier. Our 5-year project goal is to design and construct prototype fabrics comprising oligocrystalline SMA fibers.